1. Field of the Invention
The present invention relates to fuel assemblies for Pressurized Water (PWR) nuclear reactors and more particularly to such assemblies which may be easily disassembled and reassembled to permit removal and insertion or fuel or burnable poison rods within the assembly.
This invention relates to an improved joint connection between the guide tubes and the upper end fitting of a nuclear reactor fuel assembly. The design of the connection offers advantages over existing designs because or its reduced cost, the simplicity of the components, and the accomplishment of a quick disconnection procedure with fewer components.
2. Description of the Prior Art
Modern PWR fuel assemblies conventionally contain a skeleton which includes a lower end fitting, an upper end fitting and guide tubes therebetween upon which nuclear fuel rod and burnable poison rods are intermediately and spatially supported by means of a plurality of spacer grids mounted on the guide tubes.
It is known that it is advantageous to provide the fuel assemblies with guide tube/upper end fitting joints such that defective fuel rods may be replaced or burnable poison rods added after the assembly has been irradiated. This may preclude having to prematurely remove a fuel assembly from a reactor because of a few defective fuel rods. Many prior art PWR fuel assemblies have been designed with this rod replaceability in mind by providing joints between the upper end fitting and the guide tubes which may be disassembled to remove the upper end fitting and permit removal and insertion of fuel and poison rods.
The upper end fitting of a PWR nuclear fuel assembly performs several functions. It provides a grappling location for a lifting apparatus, it precisely locates the upper end of the assembly in the reactor core, and it provides reaction surfaces for springs which are used to restrain the fuel assembly from lifting under flow conditions in the reactor. As mentioned above, the upper end fitting is also designed to be remotely removable from the ends of guide tubes of the fuel assembly to provide access to the upper ends of the fuel rods.
Examples of designs to provide this latter function are shown in U.S. Pat. Nos. 4,030,975; 4,631,168; and 5,363,423. Drawbacks to all of these prior art designs are that each of the connections of the upper end fitting to the guide tubes (up to 24 joints) requires an individual disassembly operation such as unthreading, locking sleeve removal, etc. to be performed, in an irradiated environment. In some cases, U.S. Pat. No. 4,631,168, for example, small parts must be carefully contained during the periods when the joints are disassembled and trying to be reassembled.
Remote disassembly of the design shown in U.S. Pat. No. 4,030,975 involves unthreading the outer posts (thereby overcoming the resistance supplied by the distortion of the thin-walled region) and removing the upper end fitting. The outer posts sometimes require replacement at this point. Reassembly involves putting the upper end fitting back on the guide tubes, inserting and threading in the outer posts, and then re-expanding the thin-walled region of the outer posts in situ by a process that involves separate tooling.